STARBURSTS IN BARRED SPIRAL GALAXIES : II. MOLECULAR AND OPTICAL STUDY OF THREE WOLF-RAYET GALAXIES

We have searched for dense molecular gas in three barred spiral galaxies with young starbursts, NGC 3049, 5430 and 6764, which are known Wolf-Rayet galaxies. We detected HCN in the latter two, and CS was marginally detected in NGC 6764. The dense molecular gas contents of the three galaxies are compared to those of other galaxies and to other indicators of starformation.TheHCNluminosities(relativetotheCOandfar infrared ones) in these galaxies with very young starbursts are consistent with those observed in galaxies with older starbursts and in normal galaxies, and so are our upper limits to the CS intensities (relative to CO). The starburst ages evaluated from our spectrophotometric observations are in the range 3.4 to 6.0 Myr. A circum-nuclear ringisapparentonourimagesofNGC5430,thegalaxywiththe oldest central starburst; this galaxy also has the widest molecu- lar lines. The central star formation rates derived from the H luminosity are consistent with those expected from the global FIRluminosities,andarecorrelatedwiththeHCNluminosities. Finally,anindependentestimateoftheH2 columndensityis obtained by optical spectrophotometry; it leads to a H2 column density to CO intensity ratio which is about 2 to 3 times lower than the standard value, because the CO intensities of the three galaxies are higher than average, relative to their far infrared fluxes.